The store was closed for a period, and footage from the scene shows shoppers shoving each other as they attempt to grab items from trolleys.

Police were also called to the Tesco in Kingsway, Dundee, to help with crowd control.

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Media captionThe footage shows people leaping over barriers and struggling to get hold of cut-price coffee machines and televisions

Avon and Somerset Police said they were called after scuffles in the queue at Tesco in Brislington, Bristol, at about midnight.

The BBC's Zoe Conway tweeted a picture of a woman on her knees with the words: "Customers knocked to the ground in the rush at Asda Wembley #BlackFriday."

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Customers were "knocked to the ground" at Asda in Wembley

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Police were called to Tesco in Stretford, where a woman was injured by a falling television

Jamie Hook was buying food at Tesco in Stretford on Thursday night when he said "the screaming started".

"I looked at the massive crowd to see people climbing over shelves and displays, staff running for cover, fights breaking out, stock flying through air, people breaking through carrying televisions - and this was before the sale had even started," he said.

"The lady on the till I was at was in tears, terrified of it all, but she was under orders to close her till to go and help crowd control."

Ross Lewis was at one of the stores participating in Black Friday in Cardiff. He said people fought to get hold of television sets.

"People were biting each other, pinching, punching, kicking - it was just absolutely horrendous, it was so, so bad."

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Asda in Wembley was one of many UK supermarkets where Black Friday deals led to chaotic scenes

Sir Peter Fahy, Chief Constable of Greater Manchester Police, expressed his frustration via his Twitter account about the amount of Black Friday-related work being asked of his officers.

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In a statement, he also said he was "disappointed that stores did not have sufficient security staff on duty".

"Across Greater Manchester large supermarkets already make significant demands on policing through calls to shoplifting, anti-social behaviour and thefts of fuel from their petrol stations - much of which is preventable," he said.

"We just ask these stores to work with us to reduce the demands on policing and reduce the risks of disorder and crime."

"In the interest of customer safety a small number of these stores contacted police last night to help control crowds safely and stores are now trading normally."

Black Friday - historically the big sales day in America that follows the Thanksgiving holiday - has been adopted by an increasing number of shops in the UK.

The websites of several retailers, including Tesco, Argos and John Lewis, crashed earlier due to the number of shoppers trying to access them.

Despite its website problems, John Lewis said it was forecasting record online sales by the end of Friday. It said Apple's iPad mini and a discounted food processor were its fastest-selling products in early sales.